A common hydrocarbon stream to be cooled, optionally to full liquefaction, is natural gas.
Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. For example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressure.
Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.
In addition to methane, natural gas usually includes some heavier hydrocarbons and non-hydrocarbons, including but not limited to carbon dioxide, mercury, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water and other non-hydrocarbon acid gases, ethane, propane, butanes, C5+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and non-hydrocarbons either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of the non-hydrocarbons prior to the liquefying process, and reducing the levels of the heavier hydrocarbons at least for the main methane-based stream being liquefied.
Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGL) from a natural gas stream. The NGLs are usually fractionated in an NGL recovery system to yield valuable hydrocarbon products, either as products steams per se, or for use in liquefaction, for example as a component of a refrigerant or for reintroduction downstream with the main methane-based liquefied product stream.
However, NGL recovery conventionally involves cooling, condensation and fractionation steps that require significant amounts of refrigeration and other power consumption.
It is desirable to recover NGLs from a natural gas stream with the most efficient or minimal power consumption.
EP 1 469 266 A1 describes a process for the recovery of components heavier than methane from natural gas, including withdrawing from an absorber column a bottom stream enriched in components heavier than methane, and separating this into a stream containing methane and ethane, and one or more streams enriched in components heavier than ethane. However, a problem with EP 1 469 266 A1 is that its methane and ethane stream is liquefied in its main heat exchanger. This requires an extra bundle of cooling pipes in the main heat exchanger to accommodate the extra stream, and takes away cooling duty of the main heat exchanger from the main methane stream being liquefied.